Estimates of in-river concentrations (mg/l) and loads (kg/day) of nutrients to rivers in England and Wales from multiple sector sources, modelled with SAGIS (Source Apportionment GIS). The nutrients include nitrate (mg/l N) and ortho-phosphate (mg/l P); the estimate loads are expressed as kilograms per day (kg/day) and the in-river concentrations as milligrams per litre (mg/l). Sources are both diffuse and point. Diffuse sources include livestock farming, arable farming, highways, urban runoff, background (from soils), onsite wastewater treatment systems and atmospheric deposition. Point sources include treated wastewater effluent, combined sewer overflows and storm tanks, industrial discharges and mine water discharges. Concentrations and loads are modelled using the Environment Agency's catchment river model, SIMCAT, at the locations of model features or every 1 km along each river, taking into account all upstream sources and user defined river losses. SAGIS is a modelling framework was developed through the UK Water Industry Research Programme (UKWIR) project 'Chemical Source Apportionment under the WFD' [1], with support from the Environment Agency and SEPA. The model is also described in [2] [1] UKWIR (2012) Chemical Source Apportionment under the WFD (12/WW/02/3). Final report for UK Water Industry Research, 1 Queen Annes Gate, London, ISBN: 1 84057 637 5. [2] Comber, S.D.; Smith, R.; Daldorph, P.; Gardner, M.J.; Constantino, C.; Ellor, B. (2013) Development of a Chemical Source Apportionment Decision Support Framework for Catchment Management. Environ. Sci. Technol. 47, 9824-9832 Full details about this dataset can be found at https://doi.org/10.5285/8c5d9e38-0244-4a39-8600-a85513a6fecf